The most underlining reason for such difference could be emanated from the fact that these pathotypes are genetically distinct. Rice and wheat blast pathogens exhibit some differences in their chromosomal composition. Besides, there is evidence that suggested P. oryzae Tritici isolates showed relatively higher degree of SNP (Single Nucleotide Polymorphism) when compared to other strains. The variation in genetic makeup could mediate the adaptation of microbes’ macromolecules and metabolic processes to certain environmental settings (eg. temperature, humidity, host plant etc). It’s also worth noting that cross-infection between rice and wheat blast pathogens is rare in nature due to the fact that strains of certain pathogens may have distinct adaptive mechanisms to their respective ecological niches. As a result, they may show differences in host plant resistance, optimal temperature and humidity levels.

Mohammed A. Mohammed Thank you very much for your response. Just for clarification, do you think that the difference in blast disease virulence on rice and wheat at different conditions is due to the nature of the pathogen? As you've mentioned, the fungus strains are specific to crop, and here what I am considering is their virulence on the host crop.

Actually the dynamics of host-pathogen-environment interactions are vital in the process of disease development. These interactions may delicately contribute to disease optima. The genetic makeup of the pathogen may also have a huge role in the virulence potential (or mechanism) of the organism. The mechanism of virulence (against a certain host crop) such as production of virulence proteins and toxins, and also survival and production of the pathogen may be influenced by environmental conditions (CO2 concentrations, temperature & humidity). Besides, these conditions could also influence the host plant resistance pathways and defense hormone systems. When certain conditions are met for the pathogen to boost its virulence, it will develop long-lasting relationship with the host plant resulting in specificity. One way or the other, intertwined three-way interactions between the pathogen (mediated by genetic factors), the host crop, and environmental conditions could govern the extent of virulence of the pathogen.

Mohammed A. Mohammed That is great idea, thanks a lot. Above all, the points you raised on host specificity are the factors that lead to pathogen evolution with certain genetic modifications. Given that the rice blast and wheat blast are a disease caused by a pathogen (P. oryzae) but specific to different host crops, let's see why their virulence varies in different conditions. In my opinion, it is more related to the host crop's adaptation. Generally speaking, regardless of their genetic background, crops are physiologically vigor and more resistant to diseases/pests at the condition where they are accustomed to. Conversely, they become susceptible to disease/pests when there is even a slight change in their normal growth condition (it could be temperature, humidity, RF, etc). In this case, rice becomes prone to rice blast at low temperatures because it is adapted to hot conditions (summer crop) and the opposite is true for wheat (winter crop).

Thank you again for sharing your knowledge.